A systematic approach to the problem of wave scattering on plasma density irregularities is developed. The scattering process is treated as a wave field spatial evolution while crossing the scattering region, without dividing the wave field into a given incident wave and the field of the scattered waves. In such an approach, the wave energy conservation in the scattering process naturally follows from general formulas. We show that a spatial modulation of the wave packet amplitude caused by spectral broadening in k space is a general consequence of wave scattering. As an example of applications of theoretical consideration in this paper, we suggest an explanation of the amplitude modulation of VLF signals observed by the Akebono satellite in the upper ionosphere. We discuss the relation between amplitude modulation and the apparent spectral broadening of VLF transmitter signals. Both effects are described by the present theory in a similar manner. ¿ 1998 American Geophysical Union